Method for improving the oxidation-resistance of metal substrates coated with thermal barrier coatings

ABSTRACT

A method for providing a protective coating on a metal-based substrate is disclosed. The method involves the application of an aluminum-rich mixture to the substrate to form a discontinuous layer of aluminum-rich particles, followed by the application of a second coating over the discontinuous layer of aluminum-rich particles. Aluminum diffuses from the aluminum-rich layer into the substrate, and into any bond coat layer which is subsequently applied. Related articles are also described.

BACKGROUND OF THE INVENTION

[0001] This invention was made with government support under ContractNo. DEFC2195MC31176 awarded by the Department of Energy (DoE). Thegovernment may have certain rights to the invention.

[0002] This invention relates generally to protective coatings for metalsubstrates. More particularly, it is directed to improved thermalbarrier coatings applied to metals designed for high temperatureapplications.

[0003] Superalloys are often the materials of choice for componentsintended for high-temperature environments. As an example, turbineblades and other parts of turbine engines are often formed ofnickel-based superalloys because they need to maintain their integrityat temperatures of at least about 1000° C.-1150° C. Protective coatings,often referred to as thermal barrier coatings or “TBC”s, effectivelyincrease the operating temperature of turbine components by maintainingor reducing the surface temperature of the alloys used to form thevarious engine components.

[0004] Most TBC's are ceramic-based, such as a material likeyttria-stabilized zirconia. For a jet engine, the coatings are appliedto various surfaces, such as turbine blades and vanes, combustor liners,and combustor nozzles. Usually, the TBC ceramics are applied to anintervening bond layer which has been applied directly to the surface ofthe metal part. The bond layer is often critical for improving theadhesion between the metal substrate and the TBC. Bond layers areusually formed from a material like “MCrAlY”, where “M” represents ametal like iron, nickel, or cobalt.

[0005] The term “superalloy” is usually intended to embrace complexcobalt—or nickel-based alloys which include one or more other elementssuch as aluminum, chromium, tungsten, molybdenum, titanium, and iron.The quantity of each element in the alloy is carefully controlled toimpart specific characteristics, e.g., mechanical properties such ashigh-temperature strength. Aluminum is a particularly importantcomponent for many superalloys, because of its function in theprecipitation—strengthening of the alloy.

[0006] If the superalloy is exposed to an oxidizing atmosphere for anextended period of time, it can become depleted in aluminum. This isespecially true when the particular superalloy component is used at theelevated temperatures described above. The aluminum loss can occur byway of various mechanisms. For example, aluminum can diffuse into thebond coat, be consumed during oxidation of the bond coat, or be consumedduring oxidation at the bond coat/substrate interface. Thelast-mentioned mechanism is particularly severe in porous bond coats,such as air plasma-sprayed (APS) bond coats. Aluminum-loss from thesubstrate is accelerated if the TBC or bond coat is removed during theservice life of the component.

[0007] Since loss of aluminum can be detrimental to the integrity of thesuperalloy, techniques for countering such a loss have beeninvestigated. At elevated temperatures, the substrate can be partially“replenished” with aluminum which diffuses from an adjacent MCrAlY-typebond coat. However, the amount of aluminum diffusion into the substratefrom the bond coat is usually insufficient.

[0008] One method for increasing the aluminum content of the superalloyin its surface region is sometimes referred to in the art as“aluminiding”. In such a process, aluminum is introduced into thesubstrate by a variety of techniques. In the “pack aluminiding” process,the substrate is immersed within a mixture or pack containing thecoating element source, filler material, and halide energizer. Attemperatures about 850-1100° C., chemical reactions within the mixtureyield an aluminum-rich vapor which condenses onto the substrate surface,and subsequently diffuses into the substrate.

[0009] While aluminiding successfully provides aluminum to the substrateand substrate-bond coat interface, there are some disadvantagesassociated with such a technique. For example, the resultinghigh-aluminum surface layer can be brittle. Deposition of an overlaybond coat on a brittle surface can sometimes be difficult.

[0010] It should thus be apparent that new methods for increasing thealuminum content of the superalloy surface and thereby increasing itsoxidation life would be welcome in the art. These methods should preventthe formation of a brittle layer between the substrate and anysubsequently-applied layer. Moreover, the new methods should result in asurface which is very amenable to deposition of subsequently-appliedlayers. It would also be very advantageous for the new methods to becapable of providing aluminum to a bond coat layer, to compensate foraluminum consumed in the bond coat by way of oxidation.

SUMMARY OF THE INVENTION

[0011] In one embodiment, the invention is directed to a method forproviding a protective coating on a metal-based substrate, comprisingthe following step:

[0012] (a) applying an aluminum-rich mixture to the substrate to form adiscontinuous layer of aluminum-rich particles in a matrix of metallicbond coat alloy, wherein the amount of aluminum in the particles exceedsthe amount of aluminum in the metallic bond coat alloy by about 0.1atomic % to about 40 atomic %, and wherein the total amount of aluminumin the mixture is in the range of about 10 atomic % to about 50 atomicper cent.

[0013] In a second embodiment, the invention is directed to a method forproviding a protective coating on a metal-based substrate, comprisingthe following steps:

[0014] (a) applying an aluminum-rich mixture to the substrate to form adiscontinuous layer of aluminum-rich particles in a matrix of metallicbond coat alloy, wherein the amount of aluminum in the particles exceedsthe amount of aluminum in the metallic bond coat alloy by about 0.1atomic % to about 40 atomic %, and wherein the total amount of aluminumin the mixture is in the range of about 10 atomic % to about 50 atomic%; and then (b) applying at least one coating layer over thediscontinuous layer of aluminum-rich particles.

[0015] Aluminum diffuses from the aluminum-rich layer into thesuperalloy substrate, as discussed below. The discontinuous nature ofthe aluminum-rich layer prevents embrittlement.

[0016] In preferred embodiments, substantially all of the aluminum-richmaterial comprises non-oxide particles. Moreover, in many preferredembodiments, the aluminum rich layer is formed of two components.Component (I) usually comprises particles of aluminum and a secondmetal, such as nickel, while component (II) usually comprises particlesof an alloy of the formula MCrAlY, where M is a metal like Fe, Ni, Co,or mixtures which comprise any of the foregoing. The aluminum-rich layercan be applied by plasma spray techniques, such as air plasma spray orvacuum plasma spray, or by high velocity oxygen fuel (HVOF).

[0017] In some embodiments, the layer formed with the aluminum-richmixture is heat-treated after being applied, to allow diffusion ofaluminum into the superalloy. Moreover, in certain embodiments, aconventional metallic bond layer is applied over the aluminum-richlayer, prior to deposition of a thermal barrier coating. The heattreatment mentioned above can alternatively be carried out afterdeposition of the thermal barrier coating.

[0018] Another aspect of this invention is directed to an article,comprising:

[0019] (i) a metal-based substrate; and

[0020] (ii) an aluminum-containing layer over the substrate, comprisinga discontinuous layer of aluminum-rich particles. In many preferredembodiments, the article may also include a thermal barrier coatingdisposed over the aluminum-containing layer.

[0021] As mentioned previously, the aluminum-containing layer can beformed from a mixture of a component based on particles of aluminum andnickel, along with a component based on a conventional MCrAlY alloy.Moreover, a metallic bond layer can be disposed between thealuminum-containing layer and the thermal barrier coating.

[0022] Further details regarding the various aspects of this inventionare provided in the remainder of the disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]FIG. 1 is a photomicrograph of a metal substrate coated with analuminum-rich layer and a bond coat, according to the present invention.

[0024]FIG. 2 is a photomicrograph of another metal substrate coated withan aluminum-rich layer and a bond coat, according to the presentinvention.

[0025]FIG. 3 is a plot of aluminum content as a function of depth intothe bond coat and substrate, for articles based on the presentinvention.

[0026]FIG. 4 is another plot of aluminum content as a function of depthinto the bond coat and substrate, for articles based on the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

[0027] A variety of metals or metal alloys can be used as the substratefor the present invention. The term “metal-based” in reference tosubstrates disclosed herein refers to those which are primarily formedof metal or metal alloys, but which may also include some non-metalliccomponents, such as ceramics, intermetallic phases, or intermediatephases. Usually, the substrate is a heat-resistant alloy, includingsuperalloys which typically have an operating temperature of up to about1000-1150° C. They are described in various references, such as U.S.Pat. Nos. 5,399,313 and 4,116,723, both incorporated herein byreference. Illustrative nickel-base superalloys are designated by thetrade names Inconel®, Nimonic®, Rene® and Udimet®. The type of substratecan vary widely, but it is often in the form of an engine part, such asa turbine blade (bucket), a turbine nozzle guide vane, or a combustorliner. As another example, the substrate may be the piston head of adiesel engine, or any other surface requiring a heat-resistant barriercoating. In some instances, the substrate thickness can be quite small,for example, less than about 0.25 cm. Thermal protection of thin-walledsuperalloy components is often a critical task.

[0028] As mentioned above, an aluminum-rich mixture is applied to thesubstrate. Conventional pretreatment steps may be taken prior todeposition of the aluminum-rich mixture, e.g., cleaning of the substratesurface; grit blasting to remove debris and to roughen the surface; andthe like. The amount of aluminum in the aluminum-rich mixture willdepend in part on the amount of aluminum intended for diffusion from thelayer into the superalloy substrate and into any subsequently-appliedbond coat layer. Those amounts will in turn depend on the projected lossof aluminum from the substrate and bond coat layers during exposure tooxidizing atmospheres and high temperature. The approximate, projectedloss of aluminum can be determined by, first, exposing the substrate andbond coat materials in the absence of the aluminum-rich layer of thisinvention to an oxidizing environment under selected time andtemperature schedules. The microstructures of the materials can then beexamined, using various devices, such as a scanning electron microscope(SEM), equipped with an energy-dispersive X-ray detector. Such devicesare capable of quantifying the aluminum loss from the bond coat and fromthe surface region of the substrate. The “surface region” is definedherein as the region extending from the bond coat-substrate interface toabout 600 microns into the substrate.

[0029] In general, the amount of aluminum in the aluminum-rich layerwill be large enough to compensate for any projected loss of aluminumfrom the substrate or an adjacent bond coat layer, but small enough toprevent the formation of a continuous, brittle, aluminum-containinglayer, as described previously. Parameters for aluminum content, basedon atomic percentages, were provided above. In terms of weightpercentages, the amount of aluminum in the mixture is often in the rangeof about 4% by weight to about 32% by weight. In preferred embodiments,the amount of aluminum is in the range of about 10% by weight to about20% by weight. In some especially preferred embodiments, the amount ofaluminum is in the range of about 12.5% by weight to about 19% byweight.

[0030] The aluminum-rich mixture can be obtained from a variety ofsources. In general, any aluminum-containing material which can releasealuminum into adjacent surfaces or layers at elevated temperatures, suchas above about 600° C., would be suitable, as long as the material doesnot react adversely with the substrate material or the material whichforms any subsequently-applied bond coat. Non-limiting examples ofaluminum-releasing compounds which are suitable for most embodimentsinclude those based on aluminum and nickel, such as NiAl and Ni₃Al;those based on aluminum and titanium, such as TiAl and Ti₃Al; thosebased on aluminum and iron, such as FeAl and Fe₃Al; those based onaluminum and cobalt, such as CoAl; and those based on aluminum andzirconium, such as Zr₃Al. Such materials are usually commerciallyavailable, or can be prepared without undue difficulty.

[0031] In some preferred embodiments, the aluminum-rich mixture is basedon at least two components. Component (I) can be a compound based onaluminum and a second metal, as mentioned above. In general, the levelof aluminum for this component is in the range of about 20 atomic % toabout 55 atomic %, based on the total atoms of aluminum and the secondmetal. In regard to the exemplary aluminum-releasing compounds listedabove, this range generally corresponds to a weight-based range of about8% by weight to about 37% by weight aluminum.

[0032] Moreover, in preferred embodiments, the second metal is nickel toconstitute the compounds NiAl or Ni₃Al. In the case of NiAl or Ni₃Al,conversion of a preferred atomic percentage-based range to a weightpercentage-based range results in a range of about 13% by weightaluminum to about 31.5% by weight aluminum. Those skilled in the artunderstand that the proportion of atoms in such compounds may varysomewhat from the stoichiometric proportions, but are still within thescope of the formula-designation, NiAl or Ni₃Al.

[0033] In the embodiments based on at least two components, component(II) is usually a conventional material used for bond coats. Examplesinclude alloys of the formula MCrAlY, where M is selected from the groupconsisting of Fe, Ni, Co, and mixtures thereof. Many of these types ofalloys have a general composition, by weight, of about 17.0-23.0%chromium; about 4.5-12.5% aluminum; and about 0.1-1.2% yttrium; with Mconstituting the balance.

[0034] For the two-component embodiment, the relative proportions ofcomponents (I) and (II) will depend in part on some of the factorsdescribed above, regarding aluminum depletion and replenishment. Ingeneral, the level of component (I) is at least about 1% by volume,based on the total volume of components (I) and (II). In preferredembodiments, the level of component (I) is in the range of about 5% byvolume to about 50% by volume, while in especially preferredembodiments, the level of component (I) is in the range of about 15% toabout 50% by volume.

[0035] It should be understood that components (I) and (II) couldalternatively be expressed in terms of weight percentages, as long asthe densities of particles for each component are taken into account. Asan example, if component (I) is NiAl, the particle density isapproximately 5.8 g/cm³. If component (II) is NiCrAlY (having a nominalcomposition as follows: 68 wt. % Ni, 22 wt. % Cr, 9 wt. % Al, and 1 wt.% Y), its particle density is approximately 8.0 g/cm³. In such a case,1% by volume converts to about 1% by weight, based on total weight;5%-50% by volume converts to about 4%-42% by weight; and 15%-50% byvolume converts to about 11%-42% by weight. In similar fashion, theweight levels of other materials for components (I) and (II) could becalculated, based on their volume levels and densities.

[0036] The thickness of the aluminum-rich layer will also depend on avariety of factors, such as the type of substrate being protected; thepresence or absence of a subsequently-applied bond coat; and the desiredamount of aluminum content. In those instances in which no bond coat isto be subsequently applied, the aluminum-rich layer will usually have athickness in the range of about 25 microns to about 400 microns, andpreferably, in the range of about 100 microns to about 300 microns. If abond coat is to be subsequently applied, the aluminum-rich layer willoften have a thickness in the range of about 25 microns to about 300microns, and preferably, in the range of about 50 microns to about 200microns.

[0037] The aluminum-rich mixture can be applied by a variety oftechniques. Usually, a plasma spray technique or HVOF is preferred. Forthe plasma spray technique, an electric arc is typically used to ionizevarious gases, such as nitrogen, argon, helium, or hydrogen, totemperatures of about 8000° C. or greater. When the process is carriedout in an air environment, it is often referred to as air plasma sprayor “APS”. The gases are expelled from an annulus at high velocity,creating a characteristic thermal plume. Powder material is fed into theplume, and the melted particles are accelerated toward the substratebeing coated. Plasma-formed layers usually have a very rough surface,which enhances their adhesion to a subsequently-applied thermal barriercoating.

[0038] As mentioned earlier, the layer of aluminum-rich particles isdiscontinuous. In other words, the particles of the aluminum-richmaterial are not in continuous contact with each other. Instead, thesediscrete particles are usually separated from each other by particles ofanother metal-based material—usually the bond coat-type material MCrAlY.A minor portion of the aluminum-rich layer might be considered to becontiguous. “Contiguity” is a term which relates to the continuousnature of a layer of particles. Contiguity is defined herein as thefraction of total interfacial area of one phase that is shared byparticles of the same phase.

[0039] In general, contiguity is a measure of the average degree ofcontact between aluminum-rich particles in the aluminum-rich mixture.The fraction of total interfacial area of one phase shared by particlesof the same phase ranges from 0% to 100% as the distribution ofaluminum-rich phase changes from a completely dispersed structure to afully continuous structure. In some embodiments, the contiguity fractionfor the aluminum-rich particles is less than about 65%. In other words,less than about 65% of the interfacial area of each aluminum-richparticle in the layer is in contact with an adjacent aluminum-richparticle. Such a layer is said to be “discontinuous” according to thepresent description of this invention. In preferred embodiments, thecontiguity fraction for the aluminum-rich particles is less than about40%. In especially preferred embodiments, the contiguity fraction forthe aluminum-rich particles is less than about 25%.

[0040] The discontinuous nature of the aluminum-rich layer inhibits theembrittlement which characteristically occurs with continuous layers ofaluminum-rich materials. In the case of the two-component aluminum-richlayer, the aluminum-rich particles of component (I) would be separatedfrom each other by the particles of component (II), e.g., the MCrAlYmatrix.

[0041] In some embodiments and especially in the case of a superalloysubstrate, the aluminum-rich layer is heat-treated after beingdeposited. The heat treatment promotes the diffusion of aluminum intothe substrate. It is usually carried out in an oxygen-free environment,to avoid oxidation of the layer. For example, the heat treatment couldbe carried out under vacuum, in a hydrogen atmosphere, or in an inertgas atmosphere.

[0042] The particular conditions for the heat treatment will depend on avariety of factors, such as the thickness and particular composition ofthe aluminum-rich layer; the type of substrate; the projected loss ofaluminum from the substrate and/or from any subsequently-applied bondcoat layer; the projected thermal exposure for the substrate in service;the required strength for the substrate alloy; and economicconsiderations. In general, the aluminum-rich layer is usuallyheat-treated at a temperature in the range of about 925° C. to about1260° C., for a time period of about 15 minutes to about 16 hours. Insome preferred embodiments, the heat treatment is carried out at atemperature in the range of about 980° C. to about 1150° C., for a timeperiod of about 1 hour to about 8 hours. As described below, the heattreatment can alternatively be carried out at a later stage, such asafter the deposition of additional layers of various materials.

[0043] As mentioned above, a metallic bond layer or “bond coat” may beapplied over the aluminum-rich layer. The bond layer promotes adhesionbetween the coated substrate and a thermal barrier coating which isapplied at a later stage. The bond layer also provides furtherprotection to the substrate, especially in those instances in which thepart may be exposed to damaging environments. Exemplary bond coat layersare known in the art and described, for example, in U.S. Pat. Nos.5,419,971 and 5,043,138, both incorporated herein by reference. The bondcoat usually has a thickness in the range of about 50 microns to about500 microns, and preferably, in the range of about 50 microns to about375 microns. Preferred bond coatings for this invention have the formulaMCrAlY, where “M” can be various metals or combinations of metals, suchas Fe, Ni, or Co. In many instances, “M” is preferably nickel.

[0044] The bond coat may be applied by a variety of conventionaltechniques, such as plasma spray or other thermal spray depositionmethods, such as HVOF, detonation, or wire spray; CVD (chemical vapordeposition); or combinations of plasma spray and CVD techniques.Sometimes, it may be convenient to employ the same deposition techniqueused to apply the aluminum-rich layer.

[0045] In many instances, a plasma spray technique is the preferredmethod for depositing the bond layer. Specific, non-limiting examplesare low pressure plasma spraying and air plasma spraying. Detailsregarding plasma spraying can also be found, for example, inKirk-Othmer's Encyclopedia of Chemical Technology, 3rd Edition, Vol. 15,(1981) and Vol. 20 (1982); in Ullmann's Encyclopedia of IndustrialChemistry, Fifth Edition; Volume A6, VCH Publisher (1986); in ScientificAmerican, H. Herman, Sep. 1988; and in U.S. Pat. No. 5,384,200,incorporated herein by reference. One of ordinary skill in the art caneasily become familiar with various process details which may berelevant: spray distances; selection of the number of spray-passes, gastemperature; powder feed rate; powder particle size and sizedistribution; heat treatment after deposition; or any other type oftreatment after deposition.

[0046] A heat treatment as described earlier may be carried out afterdeposition of the bond coat layer. This heat treatment could replace theearlier heat treatment, or it could be an additional treatment. Theconditions for this heat treatment would be similar to those describedearlier.

[0047] The thermal barrier coating is then applied over the bond coat,or over the aluminum-rich layer in those instances in which a bond coatis not employed. Usually, the TBC is zirconia-based, as mentionedpreviously. As used herein, “zirconia-based” embraces ceramic materialswhich contain at least about 50% zirconia. In preferred embodiments, thezirconia is chemically stabilized by being blended with a material suchas yttrium oxide, calcium oxide, magnesium oxide, cerium oxide, scandiumoxide, or mixtures of any of those materials. In one specific example,zirconia can be blended with about 1% by weight to about 20% by weightyttrium oxide (based on their combined weight), and preferably, fromabout 3%-10% yttrium oxide.

[0048] The thermal barrier coating can be applied by a variety oftechniques, one being electron beam physical vapor deposition (EB-PVD).In some preferred embodiments, the thermal barrier coating is applied byplasma-spray techniques, which were described previously. Plasma spraysystems are especially suited for coating large parts, with good controlover the thickness and uniformity of the coatings. In general, thethickness of the thermal barrier coating is in the range of about 75microns to about 2000 microns. The most appropriate thickness depends inlarge part on the end use of the component.

[0049] After the thermal barrier coating has been applied, a heattreatment may be carried out, in addition to or in lieu of either of theearlier heat treatments. The conditions for this heat treatment areusually similar to those described earlier, although additional factorsare taken into consideration, such as the thickness and composition ofthe thermal barrier coating. In preferred embodiments, the heattreatment at this stage will be carried out at a temperature in therange of about 980° C. to about 1210C., for a time period of about 15minutes to about 16 hours.

[0050] As mentioned earlier, a specific heat treatment need not becarried out in some embodiments of this invention. For example, acomponent such as a turbine engine would eventually be exposed toelevated temperatures, such as about 750° C. to about 1150° C., duringits service life. Such thermal exposure would sometimes be sufficient topromote the diffusion of aluminum from the aluminum-rich layer into thesubstrate and any adjacent bond coat. The in-service heat treatment canoccur as a supplement to one or more heat treatments carried outearlier, as discussed above.

[0051] It should be apparent from the discussion set forth above thatanother aspect of this invention is directed to a metal article providedwith a protective coating, comprising:

[0052] (i) a metal-based substrate; and

[0053] (ii) an aluminum-containing layer over the substrate, comprisinga discontinuous layer of aluminum-rich particles.

[0054] In many situations, a coating layer such as a bond coat orthermal barrier coating or both (component (iii)) is applied over thealuminum-containing layer, as described previously.

[0055] The amount of aluminum in layer (ii), as applied, is usually inthe range of about 4% by weight to about 32% by weight. In preferredembodiments, the amount of aluminum is in the range of about 10% byweight to about 20% by weight. In some especially preferred embodiments,the amount of aluminum is in the range of about 12.5% by weight to about19% by weight, as described previously. The thickness of layer (ii) isusually in the range of about 25 microns to about 400 microns.

[0056] In some embodiments, a metallic bond layer is disposed betweenlayer (ii) and the thermal barrier coating layer (iii). The bond layerusually comprises an alloy of the formula MCrAlY, as described above.

[0057] Very often, the metal-based substrate is a superalloy, such as anickel-based superalloy. In those instances, the thermal barrier coatingis often zirconia-based. The superalloy may be a turbine enginecomponent, for example. The presence of the aluminum-rich layer providescritical aluminum replenishment to both the substrate and any bond coatthat has been applied before the deposition of the TBC. Thisreplenishment in turn enhances the oxidation-resistance of thecomponent.

EXAMPLES

[0058] The following examples are merely illustrative, and should not beconstrued to be any sort of limitation on the scope of the claimedinvention.

[0059] In this example, an aluminum-rich layer was first applied to aseries of superalloy substrates. Each substrate was a button made from anickel-based alloy, Rene® N-5, having a diameter of about 1 inch (2.54cm), and a thickness of about 0.125 inch (3.18 mm). Prior to depositionof the aluminum-rich layer, the coupons were cleaned with isopropylalcohol and grit-blasted.

[0060] The aluminum-rich layer was formed from two components. Component(I) was NiAl, having a nominal composition of 68.5 wt.% Ni and 31.5 wt.% Al (i.e., 50 atomic % Ni, 50 atomic % aluminum). Component (II) wasNiCrAlY, having a nominal composition as follows: 68 wt. % Ni, 22 wt. %Cr, 9 wt. % Al, and 1 wt. % Y. The particle size of components (I) and(II) was in the range of about 15 microns to about 44 microns.

[0061] For sample A (within the scope of this invention), theweight-ratio of component (I) to component (II) was 20:80. For sample B(within the scope of this invention), the weight-ratio of component (I)to component (II) was 40:60. For sample C, which was a control, theweight-ratio of component (I) to component (II) was 0:100. In otherwords, the control sample consisted of only NiCrAlY. The aluminum-richmixture was mechanically pre-mixed and air plasma-sprayed onto thesubstrate, using a standard, 45 kw plasma spray gun undernitrogen/hydrogen conditions. The following conditions were employed:Gun Power: Approximately 45 kw Gun-to-Substrate Distance: 5 inches (12.7cm) Gun Speed 1185 mm/sec (2800 in/mm) Powder Feed Rate: 6 pounds/hour(2.72 kg/hour)

[0062] The average thickness of the aluminum-rich layer was about 75microns to about 175 microns. For each sample, a bond coat was thendeposited on top of the aluminum-rich layer, using the air plasma spraysystem. The composition of the bond coat was the same as component (II)above, i.e., it was made up entirely of the NiCrAlY material. The sprayconditions were the same as those used to apply the aluminum-richmixture.

[0063] FIGS. 1 and 2 are photomicrographs of a coated substratecorresponding to Samples A and B, respectively, prior to any heattreatment of the samples. The photomicrographs were taken with a ZeissAxiovert Metallograph optical microscope. The area generally marked as“Section 1” in each figure has a depth of about 125 microns (+ or −about 20 microns), and primarily comprises NiCrAlY and voids. The areagenerally marked as “Section 2” in each figure also has a depth of about125 microns (+ or − about 20 microns), and primarily comprises a mixtureof NiCrAlY and NiAl.

[0064] In each figure, the light gray areas depicted by the arrowsrepresent particles of NiAl. It is clear that the majority of theseparticles are surrounded by the “whitish” sections which representNiCrAlY. The black area represents voids or pores within the coatingstructure.

[0065] Contiguity was measured from four fields of view, using multipletest lines, and generally following the procedure outlined in theQuantitative Stereology text mentioned above. For sample A, thecontiguity fraction was about 32% to about 42%. For sample B, thecontiguity fraction was about 50% to about 56%. Thus, in each instance,a discontinuous layer of aluminum-rich particles was present.

[0066] Standard thermal barrier coatings could be applied over thediscontinuous layer according to conventional procedures, as describedpreviously.

[0067]FIGS. 3 and 4 are plots of aluminum content as a function ofsubstrate depth and bond coat depth for articles based on the presentinvention. FIG. 3 is based on data taken prior to any heat treatment ofthe article. FIG. 4 is based on data after a vacuum heat treatment wasperformed on the article, at a temperature of about 1080° C. for 4hours.

[0068] Sample D was very similar to sample A, and was formed in the samemanner, using the same plasma spray conditions, i.e., APS-deposition ofthe same aluminum-rich layer of NiAl and NiCrAlY in a 20:80 weightratio, followed by the APS-deposition of the NiCrAlY bond coat. Sample Ewas very similar to sample B, and was formed in the same manner, usingthe same spray conditions, i.e., APS-deposition of the aluminum-richlayer of NiAl and NiCrAlY in a 40:60 weight ratio, followed by theAPS-deposition of the NiCrAlY bond coat. Sample F was similar to sampleC, i.e., a control which consisted of only NiCrAlY.

[0069] As for the previous samples, the substrate in each case was thenickel-based alloy, Rene® N-5. The nominal aluminum level in thesubstrate was 14 atomic %, while the nominal aluminum level in theNiCrAlY material was 19 atomic %. The composition profiles in thefigures represent the average of ten spot-scan profiles taken fromdifferent regions of the same sample.

[0070] In the “as-sprayed” condition (i.e., prior to heat treatment),the aluminum content of the NiCrAlY bond layer was slightly higher thanthat of the substrate alloy, but lower than that expected from thenominal composition of the NiCrAlY material. The aluminum content waslower because of surface oxidation of the particles that occurred duringAPS. Further reduction in the aluminum content in the bond coat occurredduring the heat treatment (see FIG. 4), possibly due to interdiffusionwith the underlying substrate. The resulting aluminum-compositionprofile showed very little difference in aluminum content for the bondcoat, as compared to the substrate alloy.

[0071] As shown in FIG. 3, the addition of 20 volume % NiAl to the first5 mil (about 125 microns) layer of the NiCrAlY bond coat raised thealuminum content of the bond coat slightly. Even after the heattreatment (FIG. 4), this portion of the bond coat layer contained about5 atomic % more aluminum than the substrate. The excess aluminum wouldbe available for oxidation or for interdiffusion into the substrate.

[0072] In the case of sample E (NiAl and NiCrAlY in a 40:60 weightratio), the aluminum-enrichment was more pronounced, with aluminumcontent rising as high as 30 atomic %. As in the case of sample D, thealuminum enrichment was still present after the heat treatment, and wasconcentrated in about the first 5 mil (about 125 microns) layer of theNiCrAlY bond coat.

[0073] Some of the preferred embodiments have been set forth in thisdisclosure for the purpose of illustration. However, the foregoingdescription should not be deemed to be a limitation on the scope of theinvention. Accordingly, various modifications, adaptations, andalternatives may occur to one skilled in the art without departing fromthe spirit and scope of the claimed inventive concept.

What is claimed:
 1. A method for providing a protective coating on ametal-based substrate, comprising the following step: (a) applying analuminum-rich mixture to the substrate to form a discontinuous layer ofaluminum-rich particles in a matrix of metallic bond coat alloy, whereinthe amount of aluminum in the particles exceeds the amount of aluminumin the metallic bond coat alloy by about 0.1 atomic % to about 40 atomic%, and wherein the total amount of aluminum in the aluminum-rich mixtureis in the range of about 10 atomic % to about 50 atomic %.
 2. The methodof claim 1, wherein the aluminum-rich mixture comprises particles of afirst component (component I) and particles of a second component(component II).
 3. The method of claim 2, wherein component (I)comprises particles of aluminum and a second metal, and component (II)comprises particles of an alloy of the formula MCrAlY, where M isselected from the group consisting of Fe, Ni, Co, and mixtures thereof.4. The method of claim 3, wherein the second metal for component (I) isnickel.
 5. The method of claim 3, wherein the level of component (I) isat least about 1% by volume, based on the total volume of components (I)and (II).
 6. The method of claim 5, wherein the level of component (I)is in the range of about 5% by volume to about 50% by volume, based onthe total volume of components (I) and (II).
 7. The method of claim 6,wherein the level of component (I) is in the range of about 15% byvolume to about 50% by volume, based on the total volume of components(I) and (II).
 8. The method of claim 1, wherein the discontinuous layerof aluminum-rich particles is characterized by a contiguity fraction,and the contiguity fraction is less than about 65%.
 9. The method ofclaim 1, wherein the aluminum-rich layer has a thickness in the range ofabout 25 microns to about 400 microns.
 10. The method of claim 1,wherein the aluminum-rich mixture is applied by a plasma spraytechnique.
 11. The method of claim 10, wherein the discontinuous layerof aluminum-rich particles is heat-treated.
 12. The method of claim 11,wherein the heat-treatment is carried out at a temperature in the rangeof about 925° C. to about 1260° C., for a time period between about 15minutes and about 16 hours.
 13. The method of claim 1, wherein ametallic bond layer is applied over the discontinuous layer ofaluminum-rich particles.
 14. The method of claim 13, wherein themetallic bond layer comprises an alloy of the formula MCrAlY, where M isselected from the group consisting of Fe, Ni, Co, and mixtures of any ofthe foregoing.
 15. The method of claim 14, wherein the metallic bondlayer has a thickness in the range of about 50 microns to about 500microns.
 16. The method of claim 15, wherein the metallic bond layer isapplied by a plasma spray process.
 17. The method of claim 1, whereinthe metal-based substrate is a nickel-based superalloy.
 18. The methodof claim 1, wherein a thermal barrier coating is applied over thediscontinuous layer of aluminum-rich particles.
 19. An article,comprising: (i) a metal-based substrate; and (ii) an aluminum-containinglayer over the substrate, comprising a discontinuous layer ofaluminum-rich particles.
 20. The article of claim 19, further comprisinga thermal barrier coating over aluminum-containing layer (ii).
 21. Thearticle of claim 19, wherein the amount of aluminum in layer (ii), asapplied, is in the range of about 4% by weight to about 32% by weight.22. The article of claim 19, wherein the aluminum-rich particles oflayer (ii) comprise aluminum and nickel.
 23. The article of claim 19,wherein the thickness of layer (ii), as applied, is in the range ofabout 25 microns to about 400 microns.
 24. The article of claim 19,wherein aluminum-containing layer (ii) comprises particles of a firstcomponent (component I) and particles of a second component (componentII).
 25. The article of claim 24, wherein component (I) comprisesparticles of aluminum and a second metal, and component (II) comprisesparticles of an alloy of the formula MCrAlY, where M is selected fromthe group consisting of Fe, Ni, Co, and mixtures which comprise any ofthe foregoing.
 26. The article of claim 20, wherein a metallic bondlayer is disposed between layer (ii) and the thermal barrier coating.27. The article of claim 26, wherein the bond layer comprises an alloyof the formula MCrAlY, where M is selected from the group consisting ofFe, Ni, Co, and mixtures which comprise any of the foregoing.
 28. Thearticle of claim 19, wherein the metal-based substrate is a superalloy.29. The article of claim 28, wherein the superalloy is nickel-based. 30.The article of claim 20, wherein the thermal barrier coating iszirconia-based.
 31. A method for providing a protective coating on ametal-based substrate, comprising the following steps: (a) applying analuminum-rich mixture to the substrate to form a discontinuous layer ofaluminum-rich particles in a matrix of metallic bond coat alloy, whereinthe amount of aluminum in the particles exceeds the amount of aluminumin the metallic bond coat alloy by about 0.1 atomic % to about 40 atomic%, and wherein the total amount of aluminum in the aluminum-rich mixtureis in the range of about 10 atomic % to about 50 atomic %, and then (b)applying at least one coating layer over the discontinuous layer ofaluminum-rich particles.
 32. The method of claim 31, wherein thealuminum-rich mixture comprises particles of a first component(component I) and particles of a second component (component II). 33.The method of claim 32, wherein component (I) comprises particles ofaluminum and a second metal, and component (II) comprises particles ofan alloy of the formula MCrAlY, where M is selected from the groupconsisting of Fe, Ni, Co, and mixtures thereof.
 34. The method of claim33, wherein the second metal for component (I) is nickel.
 35. The methodof claim 33, wherein the level of component (I) is at least about 1% byvolume, based on the total volume of components (I) and (II).
 36. Themethod of claim 35, wherein the level of component (I) is in the rangeof about 5% by volume to about 50% by volume, based on the total volumeof components (I) and (II).
 37. The method of claim 36, wherein thelevel of component (I) is in the range of about 15% by volume to about50% by volume, based on the total volume of components (I) and (II). 38.The method of claim 31, wherein the discontinuous layer of aluminum-richparticles is characterized by a contiguity fraction, and the contiguityfraction is less than about 65%.
 39. The method of claim 31, wherein thealuminum-rich layer has a thickness in the range of about 25 microns toabout 400 microns.
 40. The method of claim 31, wherein the aluminum-richmixture is applied by a plasma spray technique.
 41. The method of claim40, wherein the discontinuous layer of aluminum-rich particles isheat-treated.
 42. The method of claim 41, wherein the heat-treatment iscarried out at a temperature in the range of about 925° C. to about1260° C., for a time period between about 15 minutes and about 16 hours.43. The method of claim 31, wherein a metallic bond layer is appliedover the discontinuous layer of aluminum-rich particles.
 44. The methodof claim 43, wherein the metallic bond layer comprises an alloy of theformula MCrAlY, where M is selected from the group consisting of Fe, Ni,Co, and mixtures of any of the foregoing.
 45. The method of claim 44,wherein the metallic bond layer has a thickness in the range of about 50microns to about 500 microns.
 46. The method of claim 45, wherein themetallic bond layer is applied by a plasma spray process.
 47. The methodof claim 31, wherein the metal-based substrate is a nickel-basedsuperalloy.
 48. The method of claim 31, wherein a thermal barriercoating is applied over the discontinuous layer of aluminum-richparticles